In certain environments, it may be difficult to connect an electrical circuit to power supply cables, for example in hostile environments or in mechanisms in motion. To overcome this problem, micromechanical devices for converting vibration energy into electrical energy are known. These devices form microsystems generally attached to vibrating supports such as machines or vehicles. One known technique uses a resonant system to amplify a mechanical vibration of a support and convert the amplified motion into electricity. The electrical circuit can thus be powered without using cables coming from the exterior.
One of the known principles for converting mechanical vibration energy into electrical energy is based on the vibratory excitation of a beam provided with piezoelectric elements. Such a beam generally has a core with a first end embedded in a vibrating support. A mobile mass is fixed to the second end of the core. A piezoelectric element is fixed to the upper face of the core and another piezoelectric element is fixed to the lower face of the core. An electrical circuit is connected to the terminals of the piezoelectric elements which are placed electrically in series or in parallel. The core is generally made of a hard material which has a very high quality factor such as steel or silicon. The piezoelectric elements are intended for converting the mechanical energy transmitted by the mobile mass into electrical energy.
During a relative movement between the support and the mobile mass, the force of acceleration of the mobile mass induces a bending moment along the beam. This bending moment is not constant along the beam (the moment being higher at the level of the embedding then at the mass). With a core of constant section throughout its length, the mechanical stress in the upper and lower faces of the core and within the piezoelectric elements is not homogenous throughout their length, this stress being concentrated at the level of the embedding. Nor is the mechanical stress homogenous in the thickness of the core: the greater the distance from the central line (neutral fiber), the greater the mechanical stress. In the case of multilayered cores, the mechanical energy transmitted by the vibrating support towards the core is therefore distributed non-uniformly between the layers.
The use of the piezoelectric elements is limited by a maximum level, not to be exceeded, of mechanical stress which depolarizes these elements. Hence, the non-uniformity of the stress along the length of these piezoelectric elements means that the elements have to be over-sized so that a certain level of stress is not exceeded at any point, even if these elements undergo low stress along the greater part of their length. Consequently, the zones of the piezoelectric elements that undergo lower stress (for example the junction with the mobile mass) turn out to be over-sized. When there is a deformation, these less stressed zones form a parasitic capacitance receiving charges generated in the zones that are under greater stress. The quantity of electrical energy transmitted by the piezoelectric elements to the electrical circuit is thus reduced.
Besides, the piezoelectric elements are used in the longitudinal direction of the beam, corresponding to the direction along which a tensile force and/or a compressive force are applied to them during the bending of the beam. The electrical charges generated are then collected either with electrodes situated at both longitudinal ends of the piezoelectric (polarization of the piezoelectric in the longitudinal direction) or with electrodes placed on the upper and lower faces of the piezoelectric (polarization of the piezoelectric in its transverse direction). For a beam of non-negligible length, the piezoelectric elements must be sized appropriately to prevent the generation of an excessive voltage across their ends/electrodes in order to prevent the formation of electrical arcs on their edges or to avoid reaching their level of depolarizing stress. The zone of the piezoelectric elements generating the greatest amount of electricity is the zone that supports the greatest mechanical stress in proximity to the embedding. Certain conversion devices thus propose to place piezoelectric elements only at the level of the embedding.
In order to homogenize the stress in the piezoelectric elements along the length of the beam, the document drafted by Goldschmidtboeing, Müller and Woias, “Optimization of Resonant Harvesters in Piezopolymer-Composite Technology”, pages 49-51 of the document distributed at the PowerMEMS Proceedings, 28-29 Nov. 2007, describes a beam-based energy conversion device provided with a silicon core and a piezoelectric element made out of PZT attached to the upper face of the core. The silicon core and the piezoelectric element have a constant thickness but an increasing width between their ends fixed to the mobile mass and their ends fixed to the vibrating support. Thus, the stress in the piezoelectric element is homogenous along the length of the beam.
Besides, the mechanical resonance frequency of a resonance system can be modified by controlling the polarization of the piezoelectric element. The modulus of elasticity (or Young's modulus) of the piezoelectric element can thus be modified actively to modify its stiffness under compressive/tensile force and, consequently, the bending stiffness of the beam and thus influence the mechanical resonance of the resonance system. Thus, an automatic control can be set up over the mechanical resonance frequency of the system. Such automatic control can be necessary when a system from which vibration energy is extracted has variable vibration frequency. An example of such a system is a motor vehicle in which the engine rotation speed or wheel rotation speed undergoes great variations.
Such a matching of the resonance frequency, adapted to the previous example, induces a certain number of problems. The core of the beam is made out of a material having a high quality factor to reduce the mechanical damping of the conversion device but these materials (silicon, steel) generally have a high modulus of elasticity increasing the mechanical energy stored in the substrate to the detriment of the energy stored in the piezoelectric elements, thus reducing the electromechanical coupling of the complete beam and therefore its capacity to modify the resonance frequency of the structure by modification of the electrical polarization of the piezoelectric beams or the electrical load connected to it. In addition, the core is advantageously thicker than the piezoelectric elements in order to prevent the deformation of the beam from causing essentially shear stresses in the piezoelectric element, these shear stresses being unfavorable to the optimal generation of electrical voltage in the piezoelectric element. The core thus has a thickness and a modulus of elasticity that are greater than those of the piezoelectric element. Consequently, the bending stiffness of the beam is essentially defined by the bending stiffness of the core. Since the modulus of elasticity of the piezoelectric element can vary only by about 20%, the variation of the overall bending stiffness of the beam obtained by controlling the polarization of the piezoelectric element is relatively small. Consequently, the mechanical resonance frequency of the conversion device can be controlled only within a limited range. A compromise between the mechanical damping of the beam and the amplitude of its resonance frequency range is therefore necessary.
The document EP2109217 describes a tunable vibratory energy-harvesting device. The energy-harvesting device comprises a beam. The beam is provided with a main body, at least one flap and at least means to modify the shape of the flap. The flap is physically attached to the main body along a longitudinal side of the body. The shape of the flap is modified to modify the stiffness of the structure.